Generally, said engines, which are mounted in pairs on the two wings of the aircraft, are identical and have the same direction of rotation, namely the clockwise direction when viewing said engines from the rear of the aircraft.
Such a propulsion system has a certain number of advantages, in particular easier and simpler maintenance and upkeep, because of the presence of four identical engines, and also therefore of identical auxiliary means (mounting means, control means, etc.).
However, such a propulsion system has disadvantages, particularly:                an asymmetrical air flow as seen from the fuselage and the tail fin of the aircraft owing to the asymmetry produced by the wake of the propellers; and        a high noise nuisance level on the skin of the fuselage, which requires intensive and constraining noise-reduction treatments on the aircraft.        